1. Field of the Invention
The present invention relates to the operation of a broadband operational amplifier having a minimized offset voltage, and more particularly to a pair of FET operational amplifiers with capacitively coupled outputs for reducing offset errors, and having one amplifier with operation in the sub-threshold range.
2. Description of Related Art
Bipolar technology continues to offer superior analog performance over CMOS technology. Bipolar operational amplifiers have traditionally offered better gain bandwidth and lower offset voltage than field effect transistor (FET) operational amplifiers. A FET amplifier is best understood as a transconductance amplifier. Generally, an amplifier whose gain has units of conductance is referred to as a transconductance amplifier with the ratio dI.sub.out /dV.sub.in, designated as the transconductance, g.sub.m. A major drawback of the FET amplifier is that its transconductance is much lower than that of a bipolar transistor at the same current. Typically, FETs have transconductances of a few thousand micromhos at a few milliamps. In the sub-threshold region of drain current, both FETs and bipolar transistors display transconductance that increases linearly with drain (collector) current. However, the FETs transconductance is somewhat lower as compared to the bipolar transistor. As the drain current, I.sub.D, is increased above threshold into the region where I.sub.D is proportional to the square of the source-to-gate voltage, V.sub.GS.sup.2, the FET transconductance varies as the square root of I.sub.D and is well below the transconductance of a bipolar transistor at the same operating current. The transconductance, in the region above threshold, can be increased by increasing the channel width/length ratio.
Gain bandwidth performance, common to bipolar devices, can be extended to FET operational amplifiers, as described in U.S. pat. application, Ser. No. 08/855,602, entitled "COMPOUND CASCADE AMPLIFIER", filed on May 12, 1997 for Pricer, et al. Low offset voltage, however, remains a challenge to designs incorporating FET devices. Offset voltage is attributable to many sources. The dominant source for well defined amplifiers is the mismatch between the differential input pairs of transistors. Generally, bipolar input transistors can be matched to a millivolt or less, while FET transistors can be matched to only about five millivolts. In certain high gain applications, this offset voltage can exceed the signal level.
Prior art solutions that minimize the offset voltage have all required either drastically reducing the bandwidth of the operational amplifier or introducing periods of interrupted service during which the offset signal is first calibrated and then canceled.
In the pre-transistor era, the offset problem was solved with chopper stabilized amplifiers. Chopper stabilized amplifiers use high speed electromechanical relays for switching the inputs. A single coil is used to drive multiple sets of synchronized contacts. In one such technique, the contacts rapidly reverse the polarities of the inputs and outputs in synchronization so that the polarity of amplification of the signal never changes. However, since the offset can be considered lumped and fixed to one input of the amplifier, the offset voltage appears first positive and then negative at the output. A low pass filter then removes the offset voltage square wave. This technique severely limits effective amplifier operation to very low frequencies, typically, 100 Hz or less. Another drawback is the discontinuities in the output signal, caused by the rapid switching.
Another technique utilizes integrated circuits with FET capability in which the amplifier is momentarily switched out of service and then reconfigured in such a way that the offset voltage is stored in a capacitor. When the amplifier is returned to service the capacitor is typically placed in series with the input; hence, very nearly canceling out the offset. This solution is only practical, however, in applications where the amplifier does not need to be in continuous service. Examples of this technique include: Guleznski (U.S. Pat. No. 4,749,953), Dwarakanath (U.S. Pat. No. 4,306,196), Rybicki (U.S. Pat. No. 5,410,270), and Bingham (U.S. Pat. No. 4,546,324). Guleznski, in another invention (U.S. Pat. No. 4,829,263), uses a more complex scheme to minimize the switching time by using two interchangeable amplifiers.
In Vinn (U.S. Pat. No. 5,061,900) a digital version is illustrated where the offset voltage is first digitized. The Vinn invention requires less frequent calibration, but at the expense of much greater circuit complexity.
The deviations of operational amplifier voltage offset generally constitute a serious obstacle to precision circuit design and force tradeoffs in circuit configuration. Thus, designing an operational amplifier circuit with minimum offset voltage further advances amplifier selection for use in sensitive circuit applications.
The use of a FET in the sub-threshold region to minimize the offset voltage has been taught in U.S. Pat. No. 4,201,947 issued to Schade on May 6, 1980, entitled, "LONG-TAILED-PAIR CONNECTIONS OF MOSFET'S OPERATED IN SUB-THRESHOLD REGION". Schade biased MOSFET transistors by adjusting the tail current for operation in the sub-threshold region. The reduction of the tail current is achieved by increasing the resistor between two biased transistors and a remote power supply. The Schade invention achieves this result by biasing the transistor inputs only, without any further adjustment of the outputs of the FET amplifiers as part of the reduction technique.
Additionally, although the prior art seeks to subject each of the sub-portions of the broadband spectrum to special treatment such that nonlinear amplification results, based on frequency, it does not seek to produce undistorted linear amplification across the entire broadband spectrum of frequencies. In many instances, the prior art techniques also do not address the dc offset.
In U.S. Pat. No. 4,922,535 issued to Dolby on May 1, 1990, entitled, "TRANSIENT CONTROL ASPECTS OF CIRCUIT ARRANGEMENTS FOR ALTERING THE DYNAMIC RANGE OF AUDIO SIGNALS", a system of audio compression and decompression is taught which minimizes the effect of any noise introduced in a noisy transmission channel. The Dolby method breaks the audio signal into both high and low frequency portions and then treats three ranges of signal amplitude separately. Only a sub-portion of the broadband frequency is addressed, without utilizing the dc signal path.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a FET amplifier with very low offset voltage.
It is another object of the present invention to provide an amplifier that is not bandwidth limited when the offset is reduced.
A further object of the invention is to provide a circuit that extends bipolar features of low offset voltages to FET operational amplifiers.
It is yet another object of the present invention to provide an operational amplifier circuit that minimizes the offset voltage caused by mismatch between a differential input pair of transistors.
Still other advantages of the invention will in part be obvious and will in part be apparent from the specification.